Method and system for dynamically rebalancing client sessions within a cluster of servers connected to a network

ABSTRACT

A dynamic rebalancer operates in a server cluster independently of routers directing traffic to the servers in the cluster. An analysis component uses configuration information and statistics information to determine which session, if any, should be moved. A filter component receives transfer instructions from the analysis component. When the filter component receives transfer instructions from the analysis component, the filter component adds a redirect command to the client request. The filter component continually redirects client requests within the session to the new server until the client receives the redirect command and begins to send client requests to the new server.

FIELD OF THE INVENTION

The present invention deals generally with maintaining server affinitywhile load balancing requests to a server cluster. In particular, thepresent invention rebalances sessions across a server cluster havingmultiple routers without coordinating the multiple routers and whileaccounting for configuration changes in the server cluster.

BACKGROUND OF THE INVENTION

A service provider responding to client requests from a number of webbased applications needs more than one server. The service providerdistributes tasks requested by clients to applications across an arrayof individual servers, called server clusters. Clients make requests toapplications running on the individual servers in the server clusterthrough a web browser in order to receive results from the applications.The sending of requests, and the receiving of results take place in aseries of Hypertext Transfer Protocol (HTTP) communications between theclient and the server, called sessions. Examples of a session includeselecting and purchasing goods from an online retailer or performing aseries of banking transactions.

The provider of the server cluster maintains HTTP session state byemploying a mechanism so that individual HTTP clients are sent to thesame server across multiple requests in a session. When a specificserver is assigned to a specific client, the relationship between theserver and the client is called affinity, and the assigned server iscalled an affinity server.

As additional clients access the server cluster, new sessions will becreated between the clients and assigned servers. If too many sessionsare assigned to a single server, the server may become overloadedcausing a range of performance problems including system failure.Therefore, new sessions will be distributed to different servers acrossthe server cluster to balance the server load within the server cluster.The distribution of sessions across the different servers on a clusteris called load balancing.

Load balancing across multiple servers is known. Oliver Matsutti, in“Distributed Web Session Management,” (Master's Thesis, 2000) disclosesusing an HTTP Redirect command for assignment of a client's request to adifferent server from that to which the request was directed. Matsutti'ssoftware resides on the web server, and the decision to redirect therequest is made at the web server which functions as a router to selectthe destination application server.

When redirection is initiated at the router level, a problem arises whenmultiple routers are employed. In a live, distributed system, eachindividual router instance must pick the same alternative destination.In other words, different routers must have the same data at a giveninstance. Timing issues may occur in transferring the state informationnecessary for multiple routers to distribute multiple requests for agiven session to the same newly selected server. A commonly knownsolution to address such timing issues is a distributed lockingmechanism.

Specifically, the distributed locking mechanism coordinates the stateinformation in each router so that each router makes the same decisionas to where to send a request. But the distributed locking mechanismrequires extensive code to be written to coordinate the actions of therouters.

In addition to the problem of coordinating the routers, servers may beadded to or deleted from the server cluster, new applications may beinstalled on an existing server in the cluster, or the weight given aparticular server for load balancing may be changed. Therefore, anotherproblem that arises when rebalancing among individual servers in acluster is to account for changes in the server cluster configuration.

Therefore, a need exists for a mechanism to rebalance sessions acrossindividual severs in a cluster without the need to write code tocoordinate multiple routers and to account for changes in theconfiguration of the server cluster.

SUMMARY OF THE INVENTION

The system which meets the needs identified above is a dynamicrebalancer that operates in the server cluster. The DR operatesindependently of the routers so that no coordination is required betweenthe multiple routers. In a system comprising a server cluster containinga plurality of application servers and a plurality of clients connectedto the server cluster and to a plurality of routers, a dynamicrebalancer (“DR”) moves sessions without requiring coordinating code forthe routers. The DR has a configuration component (CC), a statisticcomponent (SC), a manager component (MC), an analysis component (AC),and a filter component (FC), each of which work together to accomplishload balancing within the server cluster.

The CC monitors each of the servers in the cluster and transmitsconfiguration information so that each of the servers in the clusterreceives real time information regarding the configuration of each ofthe servers in the cluster. Configuration information includes whether aserver is on line or off line, which applications are installed on eachserver, and the proportional “weight” assigned to each server. The SCregisters and receives real time statistical information for each of theservers in the cluster including the number of HTTP sessions in memoryon each server.

The MC collects the configuration information from the CC and thestatistics information from the SC and sends this information to the AC.In addition, the MC ensures that the session information, for a sessionthat is to be moved, is retrievable by other servers in the servercluster, and notifies the FC when it is safe to move the session.

The AC uses the configuration information and the statistics informationto determine which session, if any, should be moved. The AC maydetermine which session should be moved in two ways. First, respondingto a filter request from the FC, the AC may perform an analysis todetermine whether a session should be moved. Second, the AC may sendinstructions to move a session when a configuration change is detectedand the AC selects sessions for transfer to a new server.

The FC operates in two ways (corresponding to the two AC operationsdiscussed above). First the FC intercepts all client requests to aserver. The FC may send a filter request to the AC for a determinationas to whether a session should be transferred. If a determination ismade that a session should be transferred, the AC sends transferinstructions to the FC. Second, the FC receives transfer instructionsfrom the AC when the AC makes a determination in response to aconfiguration change.

When the FC receives transfer instructions from the AC, the FC adds acookie to the client request and sends the request to the new server.The cookie contains a redirect command instructing the client to sendsubsequent requests in the session to the new server. The FC continuallyredirects client requests within the session to the new server until theclient receives the redirect command and begins to send client requeststo the new server.

With the DR, all the steps to cause the redirection occur on the server,without involving the routers. The DR runs on the cluster, andtherefore, it has real-time knowledge of the cluster state.

An alternate embodiment of the DR uses a router plug-in which interceptsthe send redirect command for the duration of an active session ratherthan passing the send redirect command or a new cookie to the client.

BRIEF DESCRIPTION OF DRAWINGS

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself, however, as well asa preferred mode of use, further objectives and advantages thereof, willbe understood best by reference to the following detailed description ofan illustrative embodiment when read in conjunction with theaccompanying drawings, wherein:

FIG. 1 describes a network with a cluster of servers, multiple routersand multiple clients.

FIG. 2A depicts a storage configuration containing the dynamicrebalancer components.

FIG. 2B depicts a router storage configuration containing the alternateembodiment router plug in component.

FIG. 3 is a flowchart describing the logic of the configurationcomponent.

FIG. 4 is a flowchart describing the logic of the statistics component.

FIG. 5 is a flowchart describing the logic of the manager component.

FIG. 6 is a flowchart of the logic of the analysis component.

FIG. 7 is a flowchart of the logic of the filter component.

FIG. 8 is a flowchart of the logic of the alternative embodimentemploying a router plug-in.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The principles of the present invention are applicable to a variety ofcomputer hardware and software configurations. The term “computerhardware” or “hardware,” as used herein, refers to any machine orapparatus that is capable of accepting, performing logic operations on,storing, or displaying data, and includes without limitation processorsand memory. The term “computer software” or “software,” refers to anyset of instructions operable to cause computer hardware to perform anoperation. A “computer,” as that term is used herein, includes withoutlimitation any useful combination of hardware and software, and a“computer program” or “program” includes without limitation any softwareoperable to cause computer hardware to accept, perform logic operationson, store, or display data. A computer program may, and often is,comprised of a plurality of smaller programming units, including withoutlimitation subroutines, modules, functions, methods, and procedures.Thus, the functions of the present invention may be distributed among aplurality of computers and computer programs. The invention is describedbest, though, as a single computer program that configures and enablesone or more general-purpose computers to implement the novel aspects ofthe invention. For illustrative purposes, the inventive computer programwill be referred to as the “Dynamic Rebalancer (DR).”

Additionally, the DR is described below with reference to an exemplarynetwork of hardware devices, as depicted in FIG. 1. A “network”comprises any number of hardware devices coupled to and in communicationwith each other through a communications medium, such as the Internet. A“communications medium” includes without limitation any physical,optical, electromagnetic, or other medium through which hardware orsoftware can transmit data. For descriptive purposes, exemplary network100 has only a limited number of nodes, including client computer 1 104,client computer 2 106, client computer 3 108, client computer 4 110,router 1 122, router 2 120, server 1 132, server 2 134, server 3 136,server 4 138, and server 5 140. First network connection 102 comprisesall hardware, software, and communications media necessary to enablecommunication between network nodes 104-122. Second network connection130 comprises all hardware, software, and communications media necessaryto enable communication between network nodes 132-140. Cluster 150comprises second network connection 130, server 1 132, server 2 134,server 3 136, server 4 138, and server 5 140. Unless otherwise indicatedin context below, all network nodes use publicly available protocols ormessaging services to communicate with each other through first networkconnection 102 and second network connection 130.

Dynamic Rebalancer (DR) 200 typically is located in a storage,represented schematically as storage 202 in FIG. 2A. The term “storage,”as used herein, includes without limitation any volatile or persistentmedium, such as an electrical circuit, magnetic disk, or optical disk,in which a computer can store data or software for any duration. Asingle storage may encompass and be distributed across a plurality ofmedia. Thus, FIG. 2A is included merely as a descriptive expedient anddoes not necessarily reflect any particular physical embodiment ofstorage 202. Storage 202 resides within each of the servers of servercluster 150, or may be distributed within server cluster 150. FIG. 2Bdepicts router storage 250 with plug in 260 residing therein. Routerstorage 250 may be connected to each of the routers in network 100, ormay reside within each of the routers of network 100. As used herein,router means any software, hardware, or combination of software andhardware that functions to distribute client requests to the servers ina server cluster.

DR 200 has configuration component (CC) 300, statistic component (SC)400, manager component (MC) 500, analysis component (AC) 600, and filtercomponent (FC) 700, each of which work together to accomplish sessionload balancing within a server cluster such as server cluster 150. Inaddition, DR 200 accesses session table 220.

CC 300 monitors each of the servers in the cluster and transmits in realtime configuration information so that each of the servers in thecluster receives real time information regarding the configuration ofeach of the servers in the cluster. Configuration information includesdata indicating whether a server is on line or off line, whichapplications are installed on each server, and the proportional “weight”assigned to each server. Referring to FIG. 3, CC 300 starts 302,initializes configuration information 310, and transmits theconfiguration information to each of the other servers in the cluster(320). CC 300 monitors the cluster 330 and determines whether there hasbeen a change in the configuration of the cluster (340). If CC 300detects a change in the configuration information, it updates theconfiguration information (360) and goes to step 320. If CC 300 detectsno change, CC 300 determines whether to continue (350). If so, CC 300goes to step 330 and if not, CC 300 stops (370).

SC 400 receives and registers real time statistical information for eachof the servers in the cluster including the number of HTTP sessions inmemory on each server. Referring to FIG. 4, SC 400 starts (402) andmonitors the server cluster (410). SC 400 receives statisticsinformation from the servers on the server cluster (420) and registersthe statistics information so that it can be accessed by MC 500 (seeFIG. 5) (430). SC 400 determines whether to continue (440), and if so,goes to step 410. If not, SC 400 stops (450).

MC 500 collects the configuration information from CC 300 and thestatistics information from the SC 400 and sends this information to AC600 (see FIG. 6). Referring to FIG. 5, MC 500 starts (502), collectsconfiguration information from CC 300 (510) and transmits theconfiguration information to AC 600 (512). MC 500 collects statisticsinformation from SI 400 (514) and transmits the statistics informationto AC 600 (516). MC 500 determines whether a query has been receivedfrom FC 700 as to whether it is safe to move a session (518). If MC 500received such a query, MC 500 ensures that the session information forthe session to be moved is retrievable by other servers in the servercluster (520), and then notifies FC 700 that it is safe to move thesession (522). MC 500 determines whether to continue (540) and if sogoes to step 510 or if not, stops (550).

AC 600 uses the configuration information and the statistics informationto determine which session, if any, should be moved. AC 600 starts(602), receives configuration information from CC 300 (610), andreceives statistics information from SC 400 (612). AC 600 determineswhether a configuration change has been made to the server cluster(614). A configuration change may include, without limitation, a newserver being added, a new application being installed on one of theservers in the cluster, or a change in the weight accorded to a serverfor load balancing purposes. If a configuration change is detected, AC600 goes to step 618. If not, AC 600 determines whether a request fromFC 700 has been received (616). If so, AC 600 analyzes the configurationinformation and the statistics information (618) and determines whetherto transfer the session (620). If a determination is made to transferthe session, AC 600 sends instructions for the transfer to FC 700 andgoes to step 624. If at step 616, or at step 618, AC 600 makes anegative determination, it goes to step 624. At step 624, AC 600 makes adetermination whether to continue, and if so, goes to step 610 or ifnot, stops (626).

FC 700 starts (702) and intercepts client requests to the server cluster(710). FC 700 determines whether to send a filter request to AC 600, andif such a determination is made, sends a filter request to AC 600 (712).If not, FC 700 allows the client request to pass through to the servercluster (714) and goes to step 726. FC 700 will allow requests that arewithin ongoing active sessions to pass through, and filter requests willbe sent for new sessions. However, there are additional ways in which FC700 may be configured. For example, it may determine that theapplication to which the client request is directed is operating withinacceptable limits, or that the application is excluded from loadbalancing requirements. Thus, if the session does not need to beanalyzed, the filter allows the requests to pass through to the webapplication and, correspondingly, results to pass to the client. FC 700determines whether it has received transfer instructions from AC 600(718). If so, FC 700 adds a cookie to the corresponding client request(720) and sends the request to the new server (722). The cookie containsa redirect command instructing the client to send subsequent requests inthe session to the new server. FC 700 determines whether a request hasbeen received from the same client and session as the one for which atransfer was effected (724), and if so, goes to step 722 and sends therequest to the new server. If not, FC 700 determines whether anotherrequest has been received (726), and if so, goes to step 712 or if not,stops (730). Through step 724, FC 700 continually redirects clientrequests within the session to the new server until the client receivesthe redirect command and begins to send requests to the new server.

In an alternate embodiment, Router Plug-In Component (PI) 800 performssome of the functions of FC 700 at the router. Specifically, PI 800avoids transmitting the cookie with the send redirect command to theclient. Referring to FIG. 8, PI 800 starts (802) and monitors responsesfrom the server cluster (810). When PI 800 determines that a redirectcommand has been added to the response (812) it removes the redirectcommand (814) and forwards the response (816). If PI 800 does notdetermine that a redirect command had been added, PI 800 goes to step810. PI 800 determines whether to continue (818), and if so goes to step810 or if not stops (820).

A preferred form of the invention has been shown in the drawings anddescribed above, but variations in the preferred form will be apparentto those skilled in the art. The preceding description is forillustration purposes only, and the invention should not be construed aslimited to the specific form shown and described. The scope of theinvention should be limited only by the language of the followingclaims.

What is claimed is:
 1. An apparatus comprising: a server clusterconnected to a plurality of client computers by a plurality of routersand the Internet, the server cluster includes a plurality of servers,the client computers establish a session with the at least one of theservers in the server cluster and send client requests upon establishingthe session, the routers distribute client requests to servers in theserver cluster during the session; and a filter program residing in astorage in each of the servers of the server cluster; wherein the filterprogram, responsive to intercepting a client request from a clientcomputer to a first server in the server cluster during a session, sendsa filter request to an analysis component, and responsive to receivinginstructions to transfer the session to a second server, adds a cookieto the client request and forwards the client request to the secondserver after the session is established; wherein the analysis componentuses configuration information and a statistic information to determinewhether to transfer the session from the first server to the secondserver in order to achieve a rebalance of a workload within the servercluster; wherein the cookie contains a redirect command instructing theclient computer to send all subsequent client requests during thesession to the second server; wherein the filter program continuallyredirects each subsequent client request within the session to thesecond server until the client computer receives the redirect commandand begins to send any post command client requests during the sessionto the second server; and wherein the session is moved from the firstserver to the second server to achieve the rebalance of the workloadindependently of the plurality of routers.
 2. The apparatus of claim 1further comprising: a configuration component that transmitsconfiguration information to each of the servers in the server cluster.3. The apparatus of claim 1 further comprising: a statistics componentthat monitors the server cluster, receives and registers statisticinformation.
 4. The apparatus of claim 1 further comprising: a managercomponent that collects configuration information and statisticinformation and transmits the configuration information and thestatistic information to the analysis component.
 5. The apparatus ofclaim 4 wherein the manager component further comprises: ensuring that asession information, for a session that is to be moved, is retrievableby the plurality of servers in the server cluster.
 6. An apparatuscomprising: a server cluster connected to a plurality of clientcomputers by a plurality of routers and the Internet, the server clusterincludes a plurality of servers, the client computers establish asession with the at least one of the servers in the server cluster andsend client requests upon establishing the session, the routersdistribute client requests to servers in the server cluster during thesession; and a filter program residing in a storage in each of theservers of the server cluster; wherein the filter program, responsivereceiving instructions to transfer a session from a first server to asecond server in order to achieve a rebalance of a workload within theserver cluster, adds a cookie to the client request and forwards theclient request to the second server; wherein the filter programcontinually redirects each subsequent client request during the sessionto the second server until the client computer receives the redirectcommand and begins to send any post command client requests in thesession to the second server; and wherein the session is moved from thefirst server to the second server to achieve the rebalance of theworkload independently of the plurality of routers.
 7. The apparatus ofclaim 6 wherein an analysis component uses configuration information todetermine whether to send instructions to transfer a session to thefilter program.
 8. The apparatus of claim 6 further comprising: aconfiguration component that transmits configuration information to eachof the servers in the server cluster.
 9. The apparatus of claim 6further comprising: a statistics component that monitors the servercluster, receives and registers statistic information.
 10. The apparatusof claim 6 further comprising: a manager component that collectsconfiguration information and statistic information and transmits theconfiguration information and the statistic information to an analysiscomponent.
 11. The apparatus of claim 10 wherein the manager componentfurther comprises: ensuring that a session information, for a sessionthat is to be moved, is retrievable by the plurality of servers in theserver cluster.